This invention concerns an improvement in the selective preparation of two-carbon atom oxygenated hydrocarbons, namely acetic acid, ethanol, and/or acetaldehyde, from synthesis gas. More particularly, the invention concerns reaction of synthesis gas in the presence of a catalyst containing rhodium in combination with thorium and/or uranium to inhibit methanol formation under heterogeneous reaction conditions correlated to produce such two-carbon atom products.
The preparation of hydrocarbons and oxygenated hydrocarbons from synthesis gas (essentially a mixture of carbon monoxide and hydrogen with varying amounts of carbon dioxide) has received extensive study and has achieved commercial adoption. Reaction conditions generally involve temperatures on the order of 150.degree.-450.degree. C., pressures from atmospheric to about 10,000 psig, and hydrogen-to-carbon monoxide ratios in the range of 4:1 to about 1:4, with an iron group or a noble metal group hydrogenation catalyst.
One serious disability of most synthesis gas processes has been the non-selective or non-specific nature of the product distribution. Catalysts which possess acceptable activity generally tend to give a wide spectrum of products, for example, hydrocarbons and oxygenated hydrocarbons having a broad distribution of carbon atom contents. This not only complicates the recovery of desired products, but results in the wastage of reactants to commercially uninteresting byproducts.
Copending application Ser. No. 650,799, and U.S. Pat. No. 4,014,913 are directed to processes for selectively preparing a mixture of two-carbon atom oxygenated compounds, namely, acetic acid, ethanol, and acetaldehyde, using a rhodium catalyst. In Ser. No. 541,660, corresponding to Belgian Pat. No. 824,823 and, there are described processes for shifting the distribution of the aforementioned two-carbon atom products in favor of ethanol relative to acetic acid and acetaldehyde by incorporating elements such as iron, into the rhodium-based catalyst. A characteristic of these modified catalysts is the tendency to show an increased production of methanol which is a less valuable product than the oxygenated two-carbon atom compounds.